Concerns over the environmental impact of mankind's energy needs and use have created an increased interest in cleaner forms of energy generation. Fuel cells offer a solution to this problem due to their use of cleaner and renewable fuels, their higher energy densities and efficiencies, and their reduced emissions. A fuel cell is an energy conversion device bearing electrodes upon which electrochemical reactions occur in order to transform stored chemical energy into electrical energy. In a typical fuel cell, a fuel (for example, hydrogen gas or an alcohol) is oxidized at an anode, providing electrons that travel through an external circuit and can be used as electricity in an external load, and releasing (generally) protons, which are transported through an ion exchange membrane to a cathode side of the device. The circuit is completed within the device when the electrons reach the cathode, and combine with the protons in a reduction reaction, generally reducing oxygen to form water.
In order to improve the efficiency and selectivity of the oxidation and reduction reactions occurring on the corresponding electrodes within a fuel cell, electrocatalyst materials are typically incorporated into the electrodes. Generally, these catalysts are comprised of particulate noble metals.
The transport of protons from anode to cathode in the fuel cell is a critical and non-trivial process. The dominant method in the art that provides the appropriate scalability, generality, efficiency and manufacturing practicality is the use of polymer electrolyte membranes (PEMs). PEMs have been designed from a number of materials, but key properties that the PEM must possess are high ionic transport (of protons generally), physical robustness for manufacturing and fuel cell durability, and chemical stability to fuels and other additives. As a result, the preferred materials used for this purpose are expensive and constitute a significant portion of the materials cost of a fuel cell. Also, the membranes tend to present certain operational challenges, such as keeping the membrane sufficiently hydrated to conduct ions. While efforts are being made to lower the costs and improve performance of the PEM via a variety of methods and materials choices, the membrane still presents very significant cost and technical challenges to the overall fuel cell design.
Direct fuel cells (DFCs) use liquid fuels directly as the fuel, and a number of architectures of these are well known in the art; examples of known liquid fuels for DFCs include methanol, ethanol, other alcohols, formic acid, hydrazine, and borohydride. DFCs are simpler from a systems standpoint compared to gaseous reformate based fuel cells which require means to convert a feedstock fuel to a form usable by the fuel cell (i.e. “reformate”). DFCs also typically have a much higher volumetric energy density, and are generally safer to store, transport and refill when compared to fuel cells operating on gaseous fuel such as pure hydrogen or reformate. However there are significant performance issues with the DFCs, which include fuel cross-over, water cross-over, and poor anode and cathode catalyst kinetics. Currently, platinum group metals are typically used as catalyst materials for both half reactions. This contributes to the loss of cell voltage potential, as any fuel that crosses over into the cathode chamber will oxidize and depolarize the cathode.
Present efforts to address the performance issues with the DFC include attempts to reduce the fuel concentration of the fuel stream, to develop more fuel-tolerant cathode catalysts, and to develop membranes that are more resistant to fuel cross-over. These approaches all have drawbacks. Reducing the fuel concentration reduces the volumetric energy density of the fuel cell and is undesirable for compact power applications. Improving the membrane design is particularly challenging, as the PEM's performance is dependent on a number of factors, such as the effect of humidity on conductivity, inherent resistance of the membrane resulting in ohmic losses, and the inherent fragility of the membrane. As a result, engineering membranes with improved cross-over resistance has proven to be expensive and complex.
Fuel cells that can operate without a membrane have been reported; see for example patent application US2008/0057381. These fuel cells are known as “one pot” designs in which the anode and cathode are immersed in a mixed reactant solution, i.e. both fuel and oxidant are present in the same solution. In order for the electrodes in such fuel cells to properly function, only fuel and oxidant tolerant catalyst materials can be used. An example of one pot fuel cell design employs biological-based catalysts for one or both of the anode and cathode. These enzymatic catalysts are tolerant to the fuel and oxidant, and thus can operate in a single reaction medium without a membrane or other barrier separating the anode and cathode. Furthermore, such specialized catalyst materials present additional complexity and do not offer the performance of conventional catalyst materials such as platinum group metals